Alkyl Halides: Reactions and Properties

• These notes discuss direct halogenation, reactions involving alkenes and alcohols, and the physical and chemical properties of halides.
• It also covers nucleophilic substitution reactions (SN1 and SN2), neighboring group participation, and related concepts like nucleophilicity and basicity.

Direct Halogenation of Alkanes

• Direct halogenation of alkanes occurs in the presence of sunlight, heat, or a suitable catalyst, yielding alkyl halides.
• The reaction is represented as R-H + X2 → R-X + H-X, where X represents a halogen.
• Chlorination is fast, while bromination is slow.
• Direct iodination is not possible because the reaction is reversible and highly endothermic.
• Direct fluorination is also not possible because it is highly exothermic, leading to violent cleavage of C-C bonds.
• Direct halogenation proceeds via a free radical substitution mechanism (SN mechanism).

Reactions from Alkenes

• Halogen acids (HCl, HBr, HI) react readily with alkenes to form alkyl halides, represented as CH2=CH2 + HX → CH3-CH2X (X = Cl, Br, I).
• The order of reactivity for halogen acids is HI > HBr > HCl > HF.
• The mechanism is an addition reaction.
• Halogen acids add to unsymmetrical alkenes according to Markovnikov's rule.
• In the presence of peroxide, HBr adds to alkenes following the anti-Markovnikov rule, also known as the peroxide effect or Kharasch effect.
• This is termed Electrophilic Addu Rxn and is followed in the presence of Markoniffob. Free Radical Addn Mech followed in the presence of peroxide, that is AMR
• Allylic Substitution

Reactions from Alcohols

• Alcohols react with halogen acids (HX) to form alkyl halides and water: R-OH + HX → R-X + H2O.
• Alcohols react with PX3 to form alkyl halides and H3PO3: R-OH + PX3 → R-X + H3PO3.
• Alcohols react with PCl5 to form alkyl chlorides, POCl3, and HCl: R-OH + PCl5 → R-Cl + POCl3 + HCl.
• Alcohols react with SOCl2 (thionyl chloride) to form alkyl chlorides, SO2, and HCl (Darzen's reaction): R-OH + SOCl2 → R-Cl + SO2 + HCl.
• The byproducts of the SOCl2 reaction are gases, simplifying purification.
• The order of reactivity for halogen acids is HI > HBr > HCl, based on bond dissociation energy.
• The reactivity of alcohols follows the order 3° > 2° > 1°.
• The reaction of least reactive R-OH with HCl is carried out in the presence of anhydrous ZnCl2 (Lucas test).
• ZnCl2, being a Lewis acid, helps in the cleavage of the C-OH bond and favors the reaction by acting as a dehydrating agent.
• The reaction of 1° and 2° alcohols with HCl in the presence of ZnCl2 is called Grooves' Process.
• Tertiary alcohols readily react with concentrated HCl even in the absence of ZnCl2.
• HBr is generated in situ by the reaction of KBr or NaBr with conc. H2SO4.
• HI is generated in situ by the reaction of KI with H3PO4.
• Concentrated H2SO4 is not used when 2° or 3° alcohols are involved because they undergo dehydration to form alkenes.
• Concentrated H2SO4 is not used with HI because it is a powerful oxidizing agent and oxidizes HI to I2.
• The phosphorous halide method is better than the halogen acid method for two reasons: certain alcohols such as 2° and 3° alcohols tend to dehydrate in the presence of halogen acid, and intermediate carbocations formed can undergo rearrangement in the presence of halide.
• Darzen's process is the best method for the preparation of RCl because all the byproducts are in gaseous state and excess of sollz.

Borodine-Hunsdiecker Reaction

• R–Cl or R–Br are prepared by the action of chlorine or bromine in CCl4 on silver salts of fatty acids.
• The yield of chloroalkane or bromoalkane is usually low.
• The reaction occurs via a free radical mechanism and decarboxylation.
• This reaction gives the product with 1 less carbon atom than the fatty acid.
• Iodide forms Ester, instead of Alkyl Iodide and is called Birnbaum Simonini Rxn.

Halogen Exchange

• This is a convenient method for preparation of Alkyl Iodides (R-I).
• Finkelstein Reaction:
  • Converts alkyl halides to alkyl iodides using NaI in acetone.
• Swartz Reaction:
  • Alkyl fluorides can be obtained from corresponding chlorides or bromides by action of mercurous fluoride (Hg2F2) or antimony fluoride (SbF3) or AgF.

Physical Properties of Halides

• R-F and R-Cl are less dense, while R-Br and R-I are more dense than water.
• Density order: R-I > R-Br > R-Cl > R-F
• Boiling point order: CH3I > CH3Br > CH3Cl > CH3F, due to dipole moment, polarity, and Van der Waals forces.
• Boiling point order: CCl4 > CHCl3 > CH2Cl2 > CH3Cl.
• Dipole moment order: CH3I > CH3Br > CH3Cl > CH3F (bond length order).
• BL is proportional to Dipole Moment Order.
• Dipole moment order in the vapor phase: CH3Cl > CH3F > CH3Br > CH3I.

Chemical Properties of Halides

• Alliphatic Nu Subn Rxn

SN1 and SN2 Reactions

• SN2 reactions are bimolecular nucleophilic substitution reactions involving inversion of configuration.

Steric Effects and Leaving Groups

• A good leaving group is one that becomes a stable ion after departure and has negative charge.
• LG Ability order are :− CH3 < NH2 < OH < F. The more the stability, greater the leaving group efficeincy
• Nu Stability order are :− CH3> NH2 > OH > F. The basicity decreases as the leaving group ability increases.
• A nucleophile uses its non-bonding electrons to attack an electron-deficient atom.
• A base shares its non-bonding electrons with a proton; the strength of a base is basicity.

Factors Affecting Nu Subn Rxn

• Polar protic solvents favor SN1 reactions through solvation of leaving group.
• Polar aprotic solvents favor SN2 reactions due to increased nucleophile strength. The anionic density is high.
• Nucleophilicity is affected by basicity, steric hindrance, and solvent effects.
• SN2 reactions are concerted and stereospecific, leading to inversion of configuration.
• SN1 reactions proceed in two steps with a carbocation intermediate, leading to racemization.

Neighboring Group Participation (NGP)

• NGPs involve participation of neighboring groups in the reaction mechanism.
• Neighboring groups such as halogens, N, π bonds, or σ bonds can act as neighboring groups.
• Conditions for NGP include the presence of a lone pair containing atom, a π-bond or the σ-bond near the leaving group.